In many semiconductor device manufacturing processes, the required high levels of device performance, yield, and process repeatability can be achieved if constituents (gases, reactants, etc.) of known quality (e.g., purity) are delivered to a processing environment. Device performance, yield, and process repeatability can also be achieved if the substrate (e.g., a semiconductor wafer) remains relatively free of defects during processing of the substrate.
Rapid thermal processing (RTP) is used for several different fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). Many advances in substrate processing techniques are generally seeking higher temperature processing conditions. One high temperature process, where temperatures of 1200° C. or greater are desired is in the formation of semiconductor on insulator (SOI) substrates. SOI substrates are desired, in one regard, because such substrates offer an alternative to increased device speed and performance with a given feature size (i.e., without decreasing device feature size).
In one technique for forming an SOI substrate, two substrates are bonded together and cleaved (“bond and cleave” method). A first substrate is first subjected to oxide (e.g., SiO2) growth on a surface. A second substrate is implanted with a species that creates a damaged layer below a monocrystalline layer of sufficient thickness for device fabrication. The monocrystalline layer of the second substrate is bonded to the oxide surface of the first substrate and the composite substrate (of the first and second substrates) is cleaved at the damaged layer. The remaining composite structure includes a monocrystalline layer over an oxide layer.
The surface of the monocrystalline layer formed according to the bond and cleave method is typically rough following cleaving. The composite substrate is generally thermally annealed at a high temperature in a hydrogen (H2) atmosphere to smooth the surface. The high temperature anneal surface smoothing process consists, in one embodiment, of a soak at around 1200° C. or greater for around 30 seconds or more in a H2 atmosphere. This process is representatively described in the following articles: Sato & Yonehara, Appl Phys Lett 65 (15) 1994, pp. 1924-1926; Maszara et al., 1997 IEEE International SOI Conference Proceedings, pp. 130-131; and Moriceau et al., 1998 IEEE International SOI Conference Proceedings, pp. 37-38.
One criteria associated with high temperature anneal processing such as described is maintaining the quality of constituents delivered to the processing environment. An objective of a surface smoothing anneal, for example, is maintaining a moisture-free environment for this anneal. See Sato et al., 1998 IEEE International SOI Conference Proceedings, pp. 17-18 (noting etch or pitting rate related to amount of oxygen in hydrogen atmosphere). One guideline is that the gases used for the anneal have an oxygen or moisture content of less than one part per million (ppm). A representative purity measurement of gases that is typically delivered to a tool from a tank source, however, has been found to be on the order of 10 ppm range.
There is also a trend in RTP processes to increase substrate size so as to increase the number of devices which can be fabricated simultaneously. If substrate thickness is constant, the mass of the substrate is generally proportional to the square of its radius or edge length.
In susceptor-based processing systems, the substrate is supported by being placed on a susceptor support. Thus, the amount of support is proportional to the surface area of the substrate. In susceptorless processing systems, the substrate is generally only supported around its perimeter with an edge ring. In one type of susceptorless system, the edge ring is positioned between a heat source, such as a number of lamp heaters, and a reflector plate to reflect radiation from the heat source to a substrate seated on the edge ring. A substrate is supported at its edge by the edge ring.
In susceptorless systems, the edge ring has a tendency to impart a localized ring of scratches around the perimeter of the bottom surface of the substrate, which may be explained as follows. A substrate tends to sag where it is not supported by the edge ring, i.e., in areas away from its edge, causing the substrate to assume a curved shape. Exposure of the substrate to high temperatures makes the substrate more susceptible to sag, thus increasing its curvature.
Because of its curvature, the edge of the substrate assumes a slight angle from the horizontal. For instance, the edge of a 12-inch (300 mm) diameter substrate may be about 150 μm higher than its center at room temperature, thereby causing the edge of the substrate to assume an angle of about 0.1 degrees from the horizontal. Due to this angle, the substrate does not rest flat on the surface of the edge ring but instead contacts the ring's inside edge. As a result, the edge ring tends to scratch the bottom surface of the substrate.
300 mm (12 inch) substrates are especially susceptible to scratches for two reasons. First, 300 mm substrates are heavier and more highly curved when supported at their edge, causing the substrate to contact the edge ring with greater force. Second, larger substrates are typically provided with backside surfaces that are highly polished, which tend to show scratches more readily than unpolished surfaces.
A ridge, or “edge crown”, or nodules present on the inside edge of the edge ring were also found to scratch the substrate. The edge crown, which is formed when the edge ring is coated with a layer of polycrystalline silicon (polysilicon), is composed of excess polysilicon that preferentially deposits on the inside edge of the edge ring. The edge ring is typically coated with a layer of polysilicon to render it opaque in the frequency range used for temperature measurements of the substrate, thereby improving the accuracy of the temperature measurement. Nodules tend to form when the substrate and edge ring are of similar materials (e.g., silicon) and, as the melting point of the material is approached, the edge ring and substrate stick together and material is transferred to the edge ring. The material transferred from the substrate to the edge ring, typically classified in the form of a nodule, can damage successive substrates.
Scratches on the surface of a substrate are undesirable because they tend to increase the susceptibility of the substrate to slip. Slip is a defect in the crystalline structure of the substrate that tends to destroy any devices through which it may pass, thereby reducing the yield of the substrate. More particularly, the presence of scratches on a substrate causes slip to occur in the substrate at a lower temperature than if no scratches were present. In other words, the presence of scratches makes a substrate less robust and less able to tolerate high temperatures. Scratches also increase the susceptibility of a substrate to slip under rapidly varying temperature conditions. Scratches are therefore a particularly significant problem for substrates processed in RTP chambers, where temperatures typically exceed 1100° C. and are subject to rapid change.
In addition to increasing the susceptibility of a substrate to slip, scratches also introduce undesirable cosmetic imperfections in the substrate. Furthermore, scratches may generate stray particles that may contaminate a device fabrication process, thereby decreasing yield.